Field of the Invention
This invention relates generally to a system and method for aligning a tow hitch ball and a trailer drawbar and, more particularly, to a system and method for autonomously aligning a tow hitch ball on a towing vehicle and a trailer drawbar on a trailer through a human-machine interface (HMI) assisted visual servoing process.
Discussion of the Related Art
Some vehicles are equipped with a tow hitch that allows a trailer or other towed vehicle to be coupled thereto so that the towing vehicle can tow the trailer. Generally, the trailer hitch is mounted to a rear support structure of the towing vehicle proximate the vehicle's rear bumper, and includes a tow hitch ball having a certain diameter. The towed vehicle typically includes a trailer drawbar that extends from a front end of the towed vehicle. The trailer drawbar often includes a cup in which the hitch ball is positioned to couple the hitch to the trailer drawbar. A securing mechanism within the cup, such as a metal flap, is selectively positioned around the ball when it is inserted in the cup to securely hold the drawbar to the hitch.
When the towed vehicle is detached from the towing vehicle, the trailer drawbar is generally supported on a height adjustable stand so that the cup is positioned higher above the ground than the ball of the hitch. When the operator of the towing vehicle attaches the drawbar to the hitch, he will back up the towing vehicle to position the hitch ball just below the cup. Once in this position, the drawbar is lowered onto the ball by lowering the stand.
Generally it takes a significant amount of experience and skill for the vehicle operator to accurately position the hitch ball below the drawbar cup when backing up the towing vehicle to connect the towed vehicle to the towing vehicle. Regardless of the operator's skill and experience, it is nearly impossible to exactly position the hitch ball at the proper location. Therefore, the operator typically must use the trailer drawbar to manually move the towed vehicle in a right or left or front or back direction to provide the exact alignment. Because the towed vehicle may be large, heavy and cumbersome to move, this is sometimes a difficult task.
Modern vehicles often include one or more cameras that provide back-up assistance, provide images of the road as the vehicle is traveling for collision avoidance purposes, provide structure recognition, such as roadway signs, etc. Camera systems used for vehicle back-up assistance often employ visual overlay graphics that are super-imposed or over-laid on the camera image to provide vehicle back-up steering guidance. For those applications where graphics are overlaid on the camera images, it is critical to accurately calibrate the position and orientation of the camera with respect to the vehicle. Camera calibration typically involves determining a set of parameters that relate camera image coordinates to vehicle coordinates and vice versa. Some camera parameters, such as camera focal length, optical center, etc., are stable, while other parameters, such as camera orientation and position, are not. For example, the height of the camera depends on the load of the vehicle, which will change from time to time. This change can cause overlaid graphics of vehicle trajectory on the camera image to be inaccurate.
U.S. patent application Ser. No. 14/476,345 titled, Smart Tow, filed Sep. 3, 2014, assigned to the assignee of this application and herein incorporated by reference, discloses a system and method for providing visual assistance through a graphic overlay super-imposed on a back-up camera image for assisting a vehicle operator when backing up a vehicle to align a tow hitch ball to a trailer drawbar. The method includes providing camera modeling to correlate the camera image in vehicle coordinates to world coordinates, where the camera modeling provides the graphic overlay to include a tow line having a height in the camera image that is determined by an estimated height of the trailer drawbar. The method also includes providing vehicle dynamic modeling for identifying the motion of the vehicle as it moves around a center of rotation. The method then predicts the path of the vehicle as it is being steered including calculating the center of rotation.
The above described system in the '345 application is effective in providing visual assistance to the vehicle operator when aligning a tow ball to a trailer drawbar. However, that system requires the vehicle operator to actually control the brake, steering and throttle of the vehicle in association with the visual assistance in order to provide the alignment between the tow ball and the drawbar. Thus, there are still some drawbacks because of the operator skill level, where the ability to align the tow hitch ball with the trailer drawbar can be improved upon through a completely autonomous process.